Recent advances in a communication technology have enabled the use of transmitting and receiving sequences of very short-duration radio frequency (RF) pulses, the duration of which is typically less than a nanosecond. This is often referred to as “impulse radio (IR).”
There are various conventional implementations of receiving apparatuses and transmitting apparatuses for IR signals. IR transmitting apparatuses and IR receiving apparatuses can employ numerous data modulation (and demodulation) techniques, including amplitude modulation, phase modulation, frequency modulation, pulse-position modulation (PPM) (also referred to as “time-shift modulation” or “pulse-interval modulation”) and M-ary versions of these.
At present, the most well known method is the PPM scheme. This method works by transmitting and receiving pulses in one of a plurality of time slots in successive time frames, which requires close time synchronization between the transmitting apparatus and the receiving apparatus. The PPM modulation scheme is also referred to as a “time modulated” method, because the relativity of the arrival of the pulses is important information that the receiving apparatus must know in order to demodulate the transmission information.
One issue with the time modulated impulse radio transmission scheme is the difficulty of establishing time synchronization. This is especially true when the transmission information is comprised of bursts of transmission symbols, and, in this case, the receiving apparatus has no prior information about the timing at the transmitting apparatus.
Recently, a new alternative UWB communication scheme, referred to as a “TR (transmitted-reference) scheme” has been invented. Patent Document 1 discloses one such TR scheme. The term “transmitted-reference” refers to the transmission and reception of a plurality of pulses in such a manner that synchronization with individual pulses is unnecessary. Pulses are transmitted in pairs, and, in this case, the first pulse is a reference pulse and the second pulse is a data modulated pulse. The reference pulse and the data modulated pulse in each pulse pair are identical. Pulse pairs are separated or delayed from each other by time intervals, that is, by delays, which are known to the receiving apparatus. Transmitted-reference UWB transmits pulses in pairs, and thereby induces a correlation at the receiving apparatus that can be measured by standard means. That is, the transmitted-reference scheme does not have the synchronization problem which occurs in the time modulated scheme.
There are several methods for acquiring synchronization using the transmitted-reference scheme. Patent Document 2 discloses one such method, where a transmitted-reference, delay-hopped (TR/DH) ultra-wideband signal is used to provide a pilot signal to acquire synchronization. The term “transmitted-reference” maintains the same definition from earlier explanation. The term “delay-hopped” refers to a code-division multiple access (CDMA) scheme. Although the TR/DH scheme does not have the problem with synchronization of the time modulated scheme such as PPM/TH, the TR/DH scheme provides a poor signal-to-noise ratio (SNR) and is more likely to cause multiple access interference. According to performance prediction based on mathematical modeling and computer simulations, the influence of the multiple access interference may limit the multiple access capacity of the TR/DH scheme to a degree where it will not be suitable for some applications.    Patent Document 1: US Patent Publication No. 2003198308: “Synchronization of ultra-wideband communications using a transmitted-reference preamble.”    Patent Document 2: US Patent Publication No. 2003198212: “Method and apparatus for synchronizing a radio telemetry system by way of transmitted-reference, delay-hopped ultra-wideband pilot signal.”